Recombinant DNA technology (rDNA) has revolutionized the way therapeutic agents are prepared. The required proteins are now made inside a foreign cell and purified. Proteins having post-translational modifications (PTMs) are generally expressed as recombinant molecules in mammalian or yeast system. The yeast expression systems like Pichia and Saccharomyces are closer to mammalian systems in terms of PTMs but still differ in the types of glycosylations like high mannose glycans in case of Pichia make them unsuitable for expression of recombinant proteins for human use.
Monoclonal antibodies (mAbs), antibodies, fusion proteins, Fab fragments of mAbs are used as therapeutic agents. The rDNA technology uses specialized vectors and expression systems for production of therapeutic proteins. The expression systems mainly consist of bacterial, yeast, insect or mammalian expression systems. Initially, most of the recombinant proteins were expressed in bacterial expression system using E. coli as host. There are several advantages of using E. coli as expression host such as ease of cloning, ease of expression, shorter timelines, shorter incubation periods and very high yields. Thus, proteins which do not need any PTMs can be safely expressed in E. coli. 
Fabs, which are antigen binding fragment part of mAbs, need not to be expressed in mammalian systems as they do not contain glycosylation sites present in the Fc portion of the antibody. Hence, Fabs are usually expressed in E. coli system. During 1980-90s several researchers attempted the expression of Fabs in E. coli. PlückthunAet. al., 1990Behring Inst. Mitt. (87):48-55are some of the earlier workers who reported secretion of Fab antibody from E. coli. Williamson R. A.et. al., 1991 Biochem J. 277 (Pt 2):561-3 reported use of bacteriophage lambda vectors for expression of Fab molecules in E. coli. Phage display system for production of Fab, bivalent antibody or chimeric antibody fragments in E. coli. Moreover, Fab was also produced in E. coli as misfolded, inclusion bodies and then refolded them to get the functional molecule and thereby 40% increase in the yields of antibody was obtained.
Most of the studies mentioned above used single promoter, i.e., phoA to drive the expression of both heavy and light chains. The ribosome binding site (rbs) present in between heavy and light chains drives the transcription and translation of second gene.
U.S. Pat. No. 5,648,237 also used similar single promoter (phoA) strategy to express Fabgenes in E. colito get secreted product. The major drawback of the above strategy is that the expression levels of the second gene are usually lower than first gene, thus limiting the yields of the functional Fab.
Patent No. WO03018771 discloses a process for producing an antibody by two separate translational units, respectively encoding the light and heavy chains of said antibody or fragment, wherein both the chains are expressed in a sequential fashion, thereby specifically separating the production of the light and heavy chains and allowing the assembly of the light and heavy chains.
Patent No. EP1356052B1 discloses a method to produce full antibodies in prokaryotic cells. There is a presence of a first promoter and a first cistron to produce immunoglobulin light chain and a second promoter and a second cistron to produce immunoglobulin heavy chain, wherein both the chains are folded and assembled to form a biologically active immunoglobulin.